Parse "subscription", also prepare for type system syntax

graphql_parser/lib/src/language/ast/definition.dart

@@ -1,3 +1,9 @@
 import 'node.dart';
 
 abstract class DefinitionContext extends Node {}
+
+abstract class ExecutableDefinitionContext extends DefinitionContext {}
+
+abstract class TypeSystemDefinitionContext extends DefinitionContext {}
+
+abstract class TypeSystemExtensionContext extends DefinitionContext {}

graphql_parser/lib/src/language/ast/fragment_definition.dart

@@ -5,7 +5,7 @@ import 'package:source_span/source_span.dart';
 import 'selection_set.dart';
 import 'type_condition.dart';
 
-class FragmentDefinitionContext extends DefinitionContext {
+class FragmentDefinitionContext extends ExecutableDefinitionContext {
   final Token FRAGMENT, NAME, ON;
   final TypeConditionContext typeCondition;
   final List<DirectiveContext> directives = [];

graphql_parser/lib/src/language/ast/operation_definition.dart

@@ -6,7 +6,7 @@ import 'directive.dart';
 import 'selection_set.dart';
 import 'variable_definitions.dart';
 
-class OperationDefinitionContext extends DefinitionContext {
+class OperationDefinitionContext extends ExecutableDefinitionContext {
   final Token TYPE, NAME;
   final VariableDefinitionsContext variableDefinitions;
   final List<DirectiveContext> directives = [];
@@ -14,13 +14,18 @@ class OperationDefinitionContext extends DefinitionContext {
 
   bool get isMutation => TYPE?.text == 'mutation';
 
+  bool get isSubscription => TYPE?.text == 'subscription';
+
   bool get isQuery => TYPE?.text == 'query' || TYPE == null;
 
   String get name => NAME?.text;
 
   OperationDefinitionContext(
       this.TYPE, this.NAME, this.variableDefinitions, this.selectionSet) {
-    assert(TYPE == null || TYPE.text == 'query' || TYPE.text == 'mutation');
+    assert(TYPE == null ||
+        TYPE.text == 'query' ||
+        TYPE.text == 'mutation' ||
+        TYPE.text == 'subscription');
   }
 
   @override

graphql_parser/lib/src/language/lexer.dart

@@ -29,6 +29,7 @@ final Map<Pattern, TokenType> _patterns = {
   ')': TokenType.RPAREN,
   'fragment': TokenType.FRAGMENT,
   'mutation': TokenType.MUTATION,
+  'subscription': TokenType.SUBSCRIPTION,
   'on': TokenType.ON,
   'query': TokenType.QUERY,
   'null': TokenType.NULL,

graphql_parser/lib/src/language/parser.dart

@@ -61,7 +61,9 @@ class Parser {
     if (selectionSet != null)
       return new OperationDefinitionContext(null, null, null, selectionSet);
     else {
-      if (next(TokenType.MUTATION) || next(TokenType.QUERY)) {
+      if (next(TokenType.MUTATION) ||
+          next(TokenType.QUERY) ||
+          next(TokenType.SUBSCRIPTION)) {
         var TYPE = current;
         Token NAME = next(TokenType.NAME) ? current : null;
         var variables = parseVariableDefinitions();

graphql_parser/lib/src/language/token_type.dart

@@ -15,6 +15,7 @@ enum TokenType {
 
   FRAGMENT,
   MUTATION,
+  SUBSCRIPTION,
   ON,
   QUERY,
 

graphql_server/test/mirrors_test.dart

@@ -43,6 +43,7 @@ void main() {
   });
 }
 
+@graphQLClass
 enum RomanceLanguage {
   SPANISH,
   FRANCE,